Created attachment 153671 [details]
Sample schema and instance document exemplifying failed validation

Libxml2 fails to validate an instance document against a schema if an element whose type is a complex extension of some base type with an optional child element and that child element is not specified in the instance document.  For example, suppose I have some complex type BaseType that is defined to have one child element in a sequence group that has minOccurs set to 0:


<xs:complexType name="BaseType">
    <xs:sequence minOccurs="0">
        <xs:element name="OptionalElementInBase">
            <xs:complexType>
                <xs:attribute name="value" type="xs:string" use="required"/>
            </xs:complexType>
        </xs:element>
    </xs:sequence>
</xs:complexType>


Then say I have some other complex type DerivedType that is a complex extension of BaseType.  It adds another element that may or may not be optional like so:


<xs:complexType name="DerivedType">
    <xs:complexContent>
        <xs:extension base="BaseType">
            <xs:sequence minOccurs="0">
                <xs:element name="OptionalElementInDerived">
                    <xs:complexType>
                        <xs:attribute name="value" type="xs:string" use="required"/>
                    </xs:complexType>
                </xs:element>
            </xs:sequence>
        </xs:extension>
    </xs:complexContent>
</xs:complexType>


And then for example's sake let's say in my schema I've also declared a top level element whose type is DerivedType like so:


<xs:element name="Derived" type="DerivedType"/>


Now I also have an instance document whose root element is Derived and specify it like this (I'm leaving out the schema location attributes for brevity sake):


<Derived>
    <OptionalElementInDerived value="bar"/>
</Derived>


Running xmllint like this will then cause the above instance document to fail.


$ xmllint --schema <my schema> <my instance document>


However if I modify the instance document like so, then xmllint successfully validates the document:


<Derived>
    <OptionalElementInBase value="foo"/>
    <OptionalElementInDerived value="bar"/>
</Derived>


I have attached a tarball that has a schema and instance document that exemplifies this behavior.  Simply extracting it and running xmllint at the command line will demonstrate the problem.  I am running libxml2 2.7.6 on Red Hat Enterprise Linux 5.4.  For comparison, validation within Oxygen XML Editor using the Saxon-SA validator works just fine.  Any help would be greatly appreciated in this matter.  Thanks.